The present invention relates generally to apparatuses and methods for cleaning thin discs, such as semiconductor wafers, compact discs, glass substrates and the like. More particularly, the invention relates to cleaning the edges of a thin disc.
To manufacture a thin disc such as a semiconductor wafer, an elongated billet of semiconductor material is cut into very thin slices, about xc2xe mm in thickness. The slices or wafers of semiconductor material are then lapped and polished by a process that applies an abrasive slurry to the semiconductor wafer""s surfaces. A similar polishing step is performed to planarize dielectric or metal films during subsequent device processing on the semiconductor wafer.
After polishing, be it during wafer or device processing, slurry residue conventionally is cleaned from wafer surfaces via submersion in a tank of sonically energized cleaning fluid, via spraying with sonically energized cleaning or rinsing fluid, or via a scrubbing device which employs polyvinyl acetate (PVA) brushes, brushes made from other porous or sponge-like material, or brushes made from nylon bristles or similar materials. Although these conventional cleaning devices remove a substantial portion of the slurry residue which adheres to wafer edges, slurry particles nonetheless remain and produce defects during subsequent processing.
A conventional PVA brush scrubber disclosed in U.S. Pat. No. 5,675,856 is shown in the side elevational view of FIG. 1. The conventional scrubber 11, shown in FIG. 1, comprises a pair of PVA brushes 13a, 13b. Each brush comprises a plurality of raised nodules 15 across the surface thereof, and a plurality of valleys 17 located among the nodules 15. The scrubber 11 also comprises a platform 19 for supporting a wafer W and a mechanism (not shown) for rotating the pair of PVA brushes 13a, 13b. The platform 19 comprises a plurality of spinning mechanisms 19a-c for spinning the wafer W. During scrubbing a fluid supply mechanism F, such as a plurality of spray nozzles, supplies fluid to both major surfaces of the wafer, flushing dislodged particles and cleaning residue from the major surface of the wafer and rinsing brushes.
Preferably, the pair of PVA brushes 13a, 13b are positioned to extend beyond the edge of the wafer W, so as to facilitate cleaning the wafer""s edges. However, research shows that slurry induced defects still occur, and are caused by slurry residue remaining along the edges of the wafer despite cleaning with apparatuses such as that described above. Specifically, subsequent processing has been found to redistribute slurry residue from the wafer edges to the front of the wafer, causing defects. The same is believed to be true of all major surface cleaners, and scrubbers.
For instance, another conventional technique for cleaning slurry residue and other particles from the surfaces of a semiconductor wafer employs sonic nozzles that direct jets of liquid toward a major surface of a semiconductor wafer. FIG. 2 is a side elevational view of an exemplary sonic nozzle cleaning device 23 that includes a sonic nozzle 25 having an input port 25a, an output port 25b, and a vibrator 27 coupled to a generator 29 that drives the vibrator 27.
In operation, a cleaning solution (e.g., deionized water or another similar cleaning solution such as NH4OH, KOH, TMAH, HF, citric acid or a surfactant) is supplied under pressure (e.g., 15 p.s.i.) to the input port 25a of the nozzle 25. The cleaning solution travels through the nozzle 25, passes under the vibrator 27 and travels through the output port 25b. As the cleaning solution leaves the output port 25b it strikes the major surface of an object to be cleaned (e.g., a major surface 31a of a semiconductor wafer 31).
The vibrator 27 vibrates at a sonic rate (e.g., ultrasonic at a frequency in the hundreds of kHz or megasonic at a frequency in the thousands of kHz) set by the generator 29. As the cleaning solution travels under the vibrator 27, the vibrator 27 induces longitudinal pressure waves 33 in the cleaning solution. The longitudinal pressure-waves 33 travel to, strike and impart energy to the major surface 31a of the semiconductor wafer 31 approximately every 0.1 to 10 microseconds, depending on the particular frequency of the generator 29, thereby removing slurry residue and other particles from the major surface 31a of the wafer 31. The entire major surface 31a of the wafer 31 is cleaned by scanning the nozzle 25 across the wafer 31 while rotating the wafer 31 with a rotating mechanism 34. Slurry residue and other particles on the edges of the wafer 31, however, are not effectively cleaned by the jets of cleaning solution employed by this type of cleaning apparatus.
A number of devices have been developed to improve wafer edge cleaning. One such device is shown in the side elevational view of FIG. 3. This mechanism employs a separate edge brush 21, which is driven by a separate motor (not shown), that causes the edge brush 21 to rotate. The edge brush 21 fits over the edge of the wafer W as shown in FIG. 3, providing more effective wafer edge cleaning. Although the edge brush 21 addresses the need to clean slurry residue from wafer edges, it does so at the expense of increased scrubber complexity and cost, and the requirement of frequent edge brush replacement because of excessive mechanical wear.
Accordingly the field of wafer cleaning requires a method and apparatus which effectively cleans both the major surfaces and the edge surfaces of a semiconductor wafer, and that does so without increased cost and complexity. In short, the semiconductor processing field needs an effective edge cleaner that satisfies the ever-present demand for reduced cost per unit wafer processed.
The present invention addresses the need for an effective edge cleaner by providing a dedicated sonic nozzle specifically positioned to clean the edge surface of a thin disc such as a semiconductor wafer. A sonic nozzle (e.g., ultrasonic, megasonic, etc.) that produces a jet of liquid (e.g., de-ionized water, NH4OH, KOH, TMAH, HF, citric acid or a surfactant) is positioned so that the liquid jet strikes an edge of the thin disc to be cleaned (i.e., an edge nozzle). The sonic nozzle preferably is radially spaced from the edge of the thin disc and the liquid jet preferably is directed approximately 30xc2x0 to 150xc2x0 from a tangent to the edge of the thin disc and approximately 135xc2x0 to 225xc2x0 from a major surface of the thin disc (see FIGS. 4A-C). In this position the liquid jet impacts the edge of the thin disc, and any beveled portions thereof, with the greatest energy. Moreover the time the thin disc""s edge is exposed to sonic energy (i.e., the edge cleaning duty cycle) is significantly increased, providing superior edge cleaning. When employed with a conventional major surface cleaner, rinser or scrubber (i.e., a major surface cleaning mechanism) the invention""s edge nozzle may replace the fluid supply mechanisms conventionally required to rinse particles from a thin disc""s major surfaces, and/or to prevent thin discs such as semiconductor wafers from drying during cleaning (as drying may leave undesirable streaks and/or particles on wafer surfaces). Thus, in its preferred embodiment, the present invention cleans thin disc edges with minimal additional parts and with minimal additional cleaning fluid. Moreover, the inventive edge nozzle lasts longer than mechanical edge scrubbers, thereby reducing or eliminating replacement and maintenance costs.
To clean the entire circumference of the thin disc, the thin disc is scanned relative to the inventive edge nozzle. That is, either the thin disc is rotated while the inventive edge nozzle remains stationary, the inventive edge nozzle is scanned around the edge of the thin disc as the thin disc remains stationary, or a combination thereof.
Because the inventive edge nozzle is dedicated to edge cleaning, the nozzle may be radially spaced from the thin disc being cleaned, and therefore may be positioned so as not to obstruct or to be obstructed by a conventional major surface cleaner simultaneously employed therewith. The inventive edge nozzle thereby facilitates simultaneous use of a conventional major surface cleaner such as a brush scrubber, spin rinser (i.e., a non-scrubbing-based major surface cleaner that employs de-ionized water only), spin cleaner (i.e., a non-scrubbing-based major surface cleaner that employs a cleaning liquid such as de-ionized water and a surfactant) and even megasonic tank cleaners which clean wafers by rotating a wafer which is partially submerged in a tank of megasonically energized cleaning fluid. However, the inventive edge nozzle also may be used to clean the edge of the thin disc before or after the major surface of the thin disc has been cleaned.
Other objects, features and advantages of the present invention will become more fully apparent from the following detailed description of the preferred embodiments, the appended claims and the accompanying drawings.